In my first campaign I did three notably fantastic things.
1) I kited a Tank successfully. My tank-killer ranking was like 3200 damage done to the next guy with maybe 400 at most. I was in a warehouse and ran the fucker down two outta three aisles while emptying an auto-shotty and dual pistols into him.
2) I got that achievement for insta-gibbing a witch. We were in the sewer and she is pointblank in our only path. Being at about 30 health and having the auto-shotty, I bravely volunteer to eat her fury. Three shots later, I am a King.
3) I will cease to be a king on the climax of No Mercy when the first thing that happens is that a Smoker drags me off the roof, to the roof across from a fucking molotov impact, and helps me get maimed by 20+ zombies.
BUT MY FRIENDS HELP ME
And a hunter pins me down for another horde gang bang!
This was only normal difficulty and my friends were fantastic, though, so I survived. I did go from like.. 300 "On the Ground" health to fucking 60, though. I had to limp after a tank to help drop him.